a. Field of the Invention
This invention relates to a control system for an internal combustion engine.
It is well known to control the ignition timing and fuel mixture of an engine as a predetermined function of engine operating conditions. In modern engine control systems, the predetermined function is preprogrammed in a look-up table addressed, for example, by engine speed and load. Such a predetermined function is designed by the engine manufacturer to give a satisfactory compromise between the conflicting demands of various engine performance parameters such as levels of noxious exhaust emissions, particularly nitrogen oxides, fuel efficiency, level of combustion noise and vehicle driveability. Unfortunately, such a predetermined function cannot take account of variables which occur in use and so the desired values for the engine performance parameters are usually not achieved Examples of these variables are errors in the mixture control device, variations in fuel composition, variations between production engines and test engines, ageing effects, and changes in ambient conditions.
b. Description of the Prior Art
In order to compensate for some of these variables, it is known to measure engine outputs, such as output torque or exhaust oxygen concentration, and then provide a feedback signal for correcting ignition timing or the fuel mixture. Such a feedback signal will usually only provide compensation to the actual engine output which is measured and desired values for performance parameters related to other engine outputs will usually not be achieved.
As will now be described, various proposals have been made for making measurements on the combustion process itself and using such measurements to provide corrections to the ignition timing or fuel mixture.
In a paper entitled "Flame speed standardization between cylinders by feedback control of individual fuel injector pulse width", by T Hands et al, IMechE paper C58/88, there is described a system in which an ionization probe is located at the opposite end of a combustion chamber from the spark plug. The time which elapses between the instant of ignition (spark generation) and arrival of the flame at the probe is measured and used as a parameter to control the air to fuel ratio of the fuel mixture. Thus, this system attempts to compensate for variations in the burn rate of the fuel mixture. However, between the instant of ignition and commencement of combustion, there is a delay period which is subject to variability and so this system will not provide an accurate measurement of burn rate.
In GB-A-2 141 259, there is described a system in which burn rate is measured, with an ionization probe located in a combustion chamber. The measured burn rate is compared with a target burn rate and the resulting error is used to provide a steady state correction either to the air-to-fuel ratio or the ignition timing, but not both.
In GB-A-2 104 956, there is described a control system for a compression ignition engine provided with exhaust gas recirculation. In this system, the delay period between the initial injection of fuel and commencement of combustion is measured by using a pressure sensor located in a combustion chamber. The delay period is used as a parameter to control exhaust gas recirculation.
In unpublished United Kingdom Patent Application 8705905, there is described a system in which an optical arrangement is used to detect the crankshaft position at the start of combustion and this is used to control ignition timing.
None of these systems is capable of providing complete compensation for errors in the combustion process and so each of these systems suffers from the disadvantage that uncorrected errors in the combustion process are likely to lead to departures of engine performance parameters from their designed values.